The present invention relates to passive optical networks (PONs) and, more particularly, to a test and measurement system for detecting and monitoring losses and faults in such communication networks.
In a passive optical network (PON), optical fibers are employed in a tree or branching arrangement to distribute optical signals to a plurality of optical network units (ONUs). For example, optical signals from a central office may be transmitted, through one or more Nxc3x97N couplers, to as many as 64 ONUs. Typically, an optical time-domain reflectometer (OTDR) is used to locate faults, or to measure transmission loss in an optical fiber by launching a light pulse into the optical fiber, and then by monitoring the back-scattered signal for changes in intensity. Conventional optical time domain reflectometry, however, is generally ill-suited for passive optical networks because the branching portion of the network creates a superposition of the back-scattered signal, which cannot be readily analyzed to locate faults or to determine the transmission loss in each branch.
In the prior art, however, a xe2x80x9cbypassxe2x80x9d technique has been recently proposed for monitoring transmission loss. See, for example, U.S. Pat. No. Re 36,471 (hereinafter the xe2x80x9cRe ""471 patentxe2x80x9d), which is incorporated herein by reference. In the disclosed method of the Re ""471 patent, the optical communication network includes at least one feeder fiber for transmission between a central office, and a distribution node that is coupled to a plurality of distribution fibers, each fiber in turn coupled to an ONU. Bypass fibers, however, reroute a portion of the inbound transmission from the distribution fibers to an ODTR monitor without directly passing the transmission back through the distribution node. Transmission losses and faults are determined on the basis of delays and/or monitor wavelengths of launched optical test signals coupled into the distribution node. This method unfortunately may become costly because of the need for wavelength division multiplexing and/or active switching elements.
The present invention employs xe2x80x9cpolarization markersxe2x80x9d deployed immediately after the branching portion of a passive optical network (PON) for measuring and monitoring transmission losses and faults. Each polarization marker is configured to produce a unique polarization dependent loss (PDL) within the corresponding branch of the PON. Since each polarization marker uniquely attenuates optical test pulse(s) launched into the PON, the back-scattering uniquely varies with the launched state of polarization. Losses within each branch of the PON are then monitored by measuring the back-scattered portion of the launched optical pulse(s) as a function of time for different known states of polarization, wherein the unique PDL associated with each polarization marker is used as the basis for distinguishing the branches from one another.
In a preferred embodiment, the PON test and measurement system comprises a narrowband tunable optical source which launches short duration optical pulse(s) into the PON adjusted to have different known polarization states. As the optical pulses propagate within the PON, they encounter reflecting and scattering sites, producing an inbound optical signal which contains a superposition of the Rayleigh backscattering from each branch of PON. To deduce the transmission loss in each branch, the polarization markers are deployed immediately after branching portion of the PON so as to introduce a unique PDL in the corresponding branch. As the launched state of polarization is varied, the constituent components of the backscatter likewise varies from which the transmission loss and faults in each branch can readily be determined.
Preferably each polarization marker consists of a retarder, linear polarizer and polarization scrambler, configured to produce a unique polarization dependent loss in each branch by introducing a relative phase difference between the two constituent orthogonal polarization components of the optical pulse(s) in each branch.